Drilling tool and an indexing cutting insert for use therein

ABSTRACT

A drilling tool having an elongated body with a longitudinal axis and provided with at least one insert receiving pocket and an indexable cutting insert for use with the drilling tool for performing drilling action is disclosed. The insert of the drilling tool includes a rake face, relief flank faces, a base, a plurality of cutting edges, each defined between the rake face and adjacent relief flank face and included between two adjacent cutting corners. Each cutting edge has at least one set of component cutting edges, each set having two pairs of component cutting edges merging in a common point, each pair comprising a first and a second component cutting edge merging via a bridging section directed towards a central line constituting or parallel to a median line of the insert and passing through the common point. The component cutting edges are oriented with respect to the central line so that an outermost extremity of the second component cutting edge is closer to a center of the insert and to the central line than the outermost extremity of the first component cutting edge. This ensures that during the drilling action, a line parallel to the longitudinal axis and passing through the outermost extremity of the first component cutting edge intersects the second component cutting edge and the extremities overlap.

FIELD OF THE INVENTION

The present invention relates to a metal cutting tool having one or moreindexable cutting inserts and particularly designed for use in drilling,boring, sinking, hole formation or enlarging and similar rotary cuttingoperations. Such a tool is hereinafter generically referred to as adrilling tool.

BACKGROUND OF THE INVENTION

It is well known, in particular in drilling holes having a largedepth-to-diameter ratio, that the drilling efficiency in terms ofoutput, chip flow and tool life is significantly influenced by theefficiency of chip evacuation. In order to ensure efficient drilling andto have the chips readily removed from a cutting zone awidth-to-thickness ratio of the chip produced by a cutting edge of adrill should be kept within a certain optimum range, and to this end ithas been proposed to split the chip into relatively narrow strips.

Among such prior proposals is that disclosed in GB 20311487. Here thereare disclosed square inserts having cutting edges in which arerespectively formed pairs of chip splitting notches. Each notch hasradially innermost and outermost edges such that, when the insert ismounted in the drill in a position slightly slanted with respect to anaxis of the drill, the radially innermost edge of the outermost notch ofthe indexed cutting edge extends substantially parallel to the axis ofthe drill. The disclosure asserts that in this way chip splitting isensured. However, even if such chip splitting is achieved it is veryenergy consuming seeing that the workpiece is in continuous contact withthe notch. Besides, the construction of the cutting edge cannot providefor chip splitting over the entire length of the cutting edge as onlyone of the notches can be so positioned as to meet the requirements ofthe disclosure.

JP-59-196107 proposes a chip splitting solution similar to the onedescribed above. Here a cutting edge of an insert is formed with chipsplitting steps slightly projecting in a cutting direction. As thesesteps are also in a continuous contact with a machined workpiece, thechip separation provided by this drill is extremely energy consuming.Furthermore, in view of the fact that it is designed for drilling deepholes, the drill is provided with guide pads, making the wholeconstruction more complicated and expensive.

With drilling holes of large depth-to-diameter ratios, it is notpractical to provide indexable-insert drills with such guide pads, andas such drills are not guided by the holes it is important to ensurethat the cutting forces acting on the drill, especially radialcomponents of such cutting forces, are substantially balanced and inthis way to minimize size loads leading to tool deflection andconsequent inaccuracies in hole size.

SUMMARY OF THE INVENTION

In the further description and claims the term "center of the insert"means a center of a circle inscribed in the insert and the term "medianline" means a line drawn from the center of the insert to the center ofa line connecting two outermost cutting edge extremities.

It is the object of the present invention to provide an improved metaldrilling tool and an indexable insert for use therein, in which theabove referred-to disadvantages are substantially reduced and with whicha more effective chip splitting is ensured along the cutting edge.

According to the present invention, there is provided a metal drillingtool having an elongated body with a longitudinal axis and provided withat least one pocket at its working end, at least one indexable cuttinginsert mounted in the or each pocket for performing drilling action thesaid insert comprising: a rake face, relief flank face a base, aplurality of cutting edges, each defined between the rake face andadjacent relief flank face and included between two adjacent cuttingcorners, at least one of said cutting edges comprising at least one setof component cutting edges, each set consisting of two pairs ofcomponent cutting edges, each pair consisting of a first and a secondcomponent cutting edge, the first component cutting edges of both pairsmerging in a common point said component cutting edges being oriented sothat an extremity of the second component cutting edge innermost withrespect to said common point is closer to a center of the insert than anextremity of the first component cutting edge outermost with respect tosaid common point, said extremities merging via a bridging section whichis directed from said outermost extremity of the first component cuttingedge towards a central line constituting or parallel to a median line ofthe insert so that said innermost extremity of said second componentcutting edge is closer to said first central line than said outermostextremity of said first component cutting edge ensuring that during thedrilling action line parallel to said longitudinal axis and passingthrough said outermost extremity of the first component cutting edgeintersects said second component cutting edge and said extremitiesoverlap.

In accordance with the present invention there is also provided anindexable drilling insert for use in such a drilling tool and comprisinga rake face, relief flanks faces, a base, a plurality of cutting edges,each defined between the rake face and adjacent relief flank face andincluded between two adjacent cutting corners at least one of saidcutting edges comprising at least one set of component cutting edges,each set consisting of two pairs of component cutting edges, each pairconsisting of a first and a second component cutting edge, the firstcomponent cutting edges of both pairs merging in a common point, saidcomponent cutting edges being oriented so that an extremity of thesecond component cutting edge innermost with respect to said commonpoint is closer to a center of the insert than an extremity of the firstcomponent cutting edge outermost with respect to said common point, saidextremities merging via a bridging section which is directed from saidoutermost extremity of the first component cutting edge towards acentral line constituting or parallel to a median line of the insert sothat said innermost extremity of said second component cutting edge iscloser to said central line than said outermost extremity of said firstcomponent cutting edge ensuring overlapping of said extremities during adrilling action.

By virtue of provision of a plurality of component cutting edgesrespectively coupled together by bridging sections and all mutuallydisposed in accordance with the invention, there can be achievedeffective chip splitting for all suitable positions of the insert in thedrill.

According to the preferred embodiment of the present invention, thecutting edge of the insert is formed with one set of pairs of componentculling edges. If the cutting edge is long, it may be formed with two ormore sets of component cutting edges.

It is advantageous if the cutting edge is provided with a chamfer ateach side of the cutting corner, the chamfer merging with the secondcomponent cutting edge.

Preferably, both component cutting edges are oblique with respect to themedian line of the insert, either in the same or in differentdirections. Preferably, the first component cutting edge is slanted withrespect to the median line at an acute angle and the second componentcutting edge defines with the median line an angle which is not lessthan 90°.

According to the preferred embodiment of the present invention theinsert is provided with intermediate cutting corners located between theadjacent cutting corners of said insert so that the first componentcutting edges of both pairs of component cutting edges form sides of theintermediate cutting corner.

Preferably, the pairs of component cutting edges are symmetrical withrespect to said median line. However, an asymmetrical arrangement ofthese portions may also be advantageous.

Preferably, two or more inserts of the similar type are used in thedrill. It is advantageous if at least one of said inserts is in suchposition that its median line is slanted with respect to saidlongitudinal axis.

By virtue of the division of the insert cutting edge into the componentcutting edges, according to the present invention, and optimization ofgeometrical parameters of these component culling edges, such as theirangles with the median line, lengths and orientation with respect to thelongitudinal axis of the drill cutting forces acting on the componentcutting edges during the drilling operation, in particular the radialcomponents of the cutting forces, may be ensured to counteract and animproved force balance may be achieved in the drill. Due to the factthat the component cutting edges have different directions ofobliqueness with respect to the median line of the insert the insertitself may be provided with the improved balance of radial components ofthe cutting forces.

As mentioned above, these advantages are of extreme importance forindexable-insert drills, in particular those used for drilling holeswith large depth-to-diameter ratios, e.g. from three to six. Thusnormally the problem of the negative influence of the radialcutting-force components on the surface finish and accuracy wouldrequire subsequent machining operations so as to achieve fighttolerances or higher geometrical accuracy of the hole, which would makethe whole machining process more complicated, prolonged and expensive.Provision of the cutting edge of the indexable insert with the geometryspecified in accordance with the present invention allows for efficientdrilling of holes with large length-to-diameter ratios, especially whenan increased feed is needed, so as to ensure heavy machining ratesassociated with force/power optimization.

The rake face of the insert is preferably provided with a chip formingmeans in the form of a groove of variable dimension. It is advantageousif a depth and a width of the groove formed along a cutting edgecomponent which is outermost with respect to said longitudinal axis ofthe drill are larger than a depth and a width of the groove formed alongthe rest part of the cutting edge. The groove formed along said restpart of the cutting edge may consist of two grooves.

According to the preferred embodiment of the present invention, therelief flank face included between two adjacent cutting corners of theinsert comprises a clearance component and a support component, thecomponents sloping to the base at different angles.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding, the invention and its further preferredfeatures will now be described, by way of example only, with referenceto the following drawings in which:

FIG. 1 is a perspective view of a drilling tool in accordance with thepresent invention;

FIG. 2 is an enlarged view of the encircled detail shown in FIG. 1;

FIG. 3 is a front view illustrating disposition of the drill, showninside a workpiece during a drilling operation;

FIG. 4 is a general perspective view of an indexable insert forming partof the tool;

FIGS. 5a and 5b are respective side and top plan views of the insertshown in FIG. 4;

FIG. 6 is an enlarged schematic top plan view of a preferredcutting-edge configuration of the insert:

FIGS. 6a-6c illustrate schematically alternative configurations of thecutting edge;

FIG. 7 is an enlarged schematic end view of the insert during a drillingoperation when turned by 180°.

FIGS. 8, 9 and 10 are enlarged cross-sectional view's of the inserttaken respectively along lines VIII--VIII, IX--IX and X--X of the viewshown in FIG. 5;

FIG. 11 is a top plan view of the insert illustrating a preferred chipforming means:

FIGS. 12. 13 and 14 are cross-sectional views of the insert shown inFIG. 11 taken along the line, XII--XII, XIII--XIII and XIV--XIV;

FIG. 15 illustrates the relative disposition of a pair of drillinginserts during drilling with respect to the drilled hole;

FIG. 16 schematically illustrates the distribution of the radialcomponents of cutting forces acting upon the inserts;

FIG. 17 is another embodiment of the present invention when only oneinsert is used for enlarging a hole diameter; and

FIG. 18 is an example of a further embodiment of an insert in accordancewith the invention having a cutting edge consisting of three sets ofpairs of component cutting edges.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIGS. 1, 2 and 3 show a drilling tool in accordance with the presentinvention, which is to be used in drilling a hole in a workpiece W of adiameter D and having a large depth-to-diameter ratio, e.g. from 3 to 6.As seen, the drill, having a longitudinal axis Z, comprises a body 1provided with a shank 2 to be held by an adaptor attached to a machinespindle (both not shown). In a portion of the drill body adjacent to aworking end 3 there are formed flutes 4 and 4'. The body of the drillmay also be provided with through-going passages 5 and 5' for a coolingmedium.

At the working end 3 of the body recessed pockets 6 and 6' are providedfor replaceably mounting, therein respective indexable cutting inserts 7and 7'. As seen in FIGS. 1 and 2, each pocket 6, 6' has an appropriatebase face 8 and side surfaces 10 and 12 for supporting the insert whichis secured in position by a screw 14. It is common with drills of thistype that the inserts are arranged in the drill so that a radially,innermost insert 7' (FIG. 3') is situated adjacent a central region ofthe drill body and a radially outermost insert 7 is spaced from thedrill axis Z so that the distance between the outermost extremity of thecutting edge and the axis Z defines the diameter D of the hole drilledin the workpiece W. Such an arrangement enables one insert to cut acentral portion of the hole and the other--the peripheral portion.

In the preferred embodiment both inserts have similar construction andtherefore only one insert 7 will be further described with reference toFIGS. 4, 5a and 5b, where it is seen to be of the trigon type with ahexagonal geometry, i.e. with three main cutting corners 18, 20 and 22and three intermediate corners 24, 26 and 28.

The insert 7 is provided with a rake face 32, relief flanks 34, 36, 38(two of them are not seen) and a base 40. The insert 7 is so mountedthat the rake face 32 of the insert preferably defines an axial angle(not shown) with an axis parallel to the axis Z. The rake surface 32 ofthe insert is formed with an appropriate chip forming means which willbe further explained in more detail.

The relief flank 34 has a design similar to the relief flanks 36 and 38and consists of relief flank portions 34a, 34b, 34c, where the portion34a is associated with each main cutting corner, the portion 34c isassociated with each intermediate cutting corner and the portion 34b isincluded between adjacent main and intermediate cutting corners. Therelief flank portions 34a, 34b and 34c comprise two successive reliefflank components 34' and 34" shown in FIG. 7 and also seen in FIGS.8-10, which slope with respect to the base 40 at different angles α1 andα2. The upper relief flank component 34' serves as a clearance componentand the lower relief flank component 34" is used for the insert support,when the insert is mounted in the tool. Besides. the relief flankportion 34a is provided with an additional relief flank component 34"'sloping with respect to the base at the angle α3 (shown in FIG. 8). Byvirtue of sloping angles α1, α2 and α3, appropriate clearance betweenthe insert and an inner side wall of the drilled hole is provided.

An intersection of the relief flanks with the rake face defines cuttingedges 50, 60 and 70 each included between a respective pair of adjacentmain cutting corners. Each cutting edge consists of two similar pairs ofcomponent cutting edges. With reference to FIG. 6, the construction ofonly one pair of component cutting edges of only one cutting edge willbe explained, but it should be understood that the other componentcutting edges have, preferably, a similar geometry.

Thus, the cutting edge 50 shown in FIG. 6 is included between the twoadjacent main cutting corners 18 and 20 and consists of two pairs50a-50b and 50a'-50b' of component cutting edges so that the componentcutting edges 50a and 50a merge in a common point constituted by theintermediate cutting corner 24.

The component cutting edge 50a has an extreme point A which is itsoutermost extremity with respect to the intermediate cutting corner 24and the component cutting edge 50b has an extreme point B which is itsinnermost point with respect to the intermediate corner 24. Thecomponent cutting edges 50a and 50b are so disposed that the point B ofthe component cutting edge 50b is closer to a center C of the insertthan the point A of the component cutting edge 50a and the two pointsmerge smoothly via a bridging section S. The section S is so inclinedwith respect to a median line M that the point B is closer to the medianline than the point A and a line Z' parallel to the axis Z and passingthrough the point A of the first component cutting edge that the line Z'intersects the second component cutting edge 50b ensuring, anoverlapping of extremities of the two component cutting edges adjacentto their extreme points A and B during the drilling operation.Therefore, an angle 52 between the bridging section S and the medianline M should be chosen of such a value as to prevent the bridgingsection S from being parallel to the line Z' and the workpiece W fromcontacting the extremity of the component cutting edge 50b adjacent toits innermost point B, thereby enabling an effective chip splittingduring the drilling action. It is also important for the angle 52 to besmall enough so as not to weaken the cutting edge adjacent the point Aof the component cutting edge 50a. In practice, this angle is in therange from 15° to 35°. A length of the bridging section defined by itsprojection L on the line M should be chosen so to ensure the effectivechip splitting. In practice, this value should be not less than amaximal feed, e.g., 0.5:2 times the maximum feed, along axis Z asmeasured in ram/rev. The preferred length of the projection L is from0.2 to 0.6 mm.

The orientation of the component cutting edges 50a and 50b is defined byrespective angles 54 and 56 formed by these component cutting edges (incase the of component cutting edge 50b --by its continuation) with theline M. In the preferred embodiment the component cutting edges 50a and50b are slanted with respect to the line M so that the angle 54 is acuteand angle 56 is not less than 90°, the angles being measured on the sideof the center C of the insert. In the described preferred embodiment theangle 54 is half a penetration angle defined by the intermediate cuttingcorner 24. In practice, the required value of the penetration anglewhich depends mostly on a drilled material, is in the range of120°:160°. Thus, the angle 54 is in the range of 60°:80°. The value ofangle 56 is dictated by a compromise between the requirements to thecomponent cutting edge 50b to be inclined, on the one hand, to provide aforce balance and, on the other hand, not to weaken the adjacent cuttingcorner 20. The angle 56 is, preferably, in the range of 90°:120°. Itwill be further explained that the specific disposition of the componentcutting edges 50a and 50b in accordance with this preferred embodimentensures the component culling edges to be subject to different cuttingforces, in particular to radial components of the culling forces,generated during the drilling action.

The projections 46, 48 of the component culling edges 50a and 50b on aline H normal to the axis Z actually define the width of a chip producedby the corresponding component cutting edge during the drilling action.The relative lengths of these projections are to be chosen so as toensure, on the one hand, optimum width of the split chips and on theother hand, a minimum unbalanced forces acting on the tool. Thus, in thepreferred embodiment the projection 48 of the component cutting edge 50bis larger than the projection 46 of the component cutting edge 50a. Inpractice, the ratio between the projections 46 and 48 is about 1:1.5. Inthe preferred embodiment both component cutting edges are straight. Itshould be mentioned, however, that one component cutting edge or both ofthem may be curved and/or have a wavy configuration.

The cutting edge 50 is preferably provided with chamfers situated atboth sides of the main cutting corner 20. A chamfer WW facing the faceof the drilled hole merges the second component cutting edge 50b andtakes part in cutting process, while the chamfer facing the inner sidewall of the hole acts as a wiper. The provision of such chamfers allows,on the one hand, to advantageously increase an angle 58 adjacent to theend of the component cutting edge 50b and therefore to strengthen themain cutting corner 20 and, on the other hand, to improve the insertstability with respect to side wall of the hole which in the combinationwith the above-described configuration of the cutting edge enablesefficient drilling and results in improved accuracy and surface finishof the drilled hole. In the preferred embodiment the value of the angle58 lies in the range 38°:70°.

It is seen in FIG. 7 that when the insert 7 rotates about the axis Z thepoint A of component cutting edge 50a describes a circular path C' ofradius R' concentric to a circular path C of radius R described by thecorner 20. It is seen that in this case the point A of the cutting edgeworks under conditions of internal cutting, anti therefore theappropriate clearance F' should be provided between the respectiveportion 34' of the relief flank component 34c of the insert and theworkpiece W. In order to achieve this clearance F' the relief flank 34cassociated with the intermediate cutting corner 24 is provided by a step80 formed along the direction of thickness of the insert which isdefined by lines 80' and 80", sloping with respect to insert base 40 atangles δ1 and δ2. where δ1>δ2.

With reference to FIGS. 8-10, a general construction of a chip formingmeans provided on the insert rake face 32 will be described. This meansis formed as a groove 90 extending along the cutting edges of theinsert. The chip forming groove 90 comprises an initial land surface 91,91', 91" situated adjacent to the insert tuning edge and followed by adownward sloping portion 92, 92', 92", which in turn merges with anuprising chip deflection portion 93, 93', 93". The depth of the chipforming groove varies along the length of the cutting edge so that inthe vicinity of the main cutting corner which is outermost with respectto the axis Z (see FIG. 6) the groove has its largest depth 94, at thevicinity of the intermediate cutting corner the groove has anintermediate depth 94" and between the intermediate culling corner andthe main cutting corner which is innermost with respect to the axis Zthe groove has its smallest depth 94'. It is advantageous if the slopeof the land surface in each cross-section of the insert is such as toensure a local rake angle included between the land surface and a lineR, R', R" to remain substantially invariant along the entire cuttinginsert perimeter. The rake angle can be positive or negative

Preferably, the chip forming means is provided with additional specificfeatures, illustrated in FIGS. 11, 12, 13 and 14, where the chip forminggroove 90 is shown to have different shape in the vicinity of thedifferent component cutting, edges depending on a remoteness of thecomponent cutting edge from the axis of the drill when the insert ispositioned therein. Thus, the shape of the chip forming groove 90 issuch that in a central part of the cutting edge, i.e. in the vicinity ofthe intermediate cutting corner 24, and in aside part between theintermediate cutting corner 24 and a radially innermost main cuttingcorner 18 the chip forming groove consists of two grooves 95 and 96having different depths h1 and h2 and extending from the cutting edge atdistances L1 and L2, while in the vicinity of the radially outermostmain cutting corner 20 the chip forming groove consists of one largegroove 97 with a depth h3>h1;h2 and a width L3>L2. By virtue of theabove-described geometry of the chip forming groove, account is taken ofthe influence on chip formation of the cutting speed varying along thecutting edge and an efficient chip control and easy evacuation of chipsat low and medium feeds is ensured, providing thereby an increasedoutput of the drilling operation.

In FIG. 15 there is illustrated a relative disposition of the cuttinginserts when the drill rotates about its axis Z, where the position ofeach cutting insert in accordance with FIG. 3 is shown in full lines anda fragment of the position of each insert after turning the drill by180° is shown in dotted lines. As can be seen, a profile P is thuscreated on a face of the hole corresponding to respective projections100 and 100' of the cutting edges 50 and 50' of the inserts, whichprojections overlap providing a continuous cutting across the radialregion included between the drill axis and the end of the cutting edgeof the radially outermost insert 7. The profile P includes annularportions Q which max serve as guiding means and improve concentricityand stability of the drill. If desired, the annular portions Q of theprofile P may be deliberately reduced or eliminated by slightly slantingone or both of the inserts and by a proper choice of a radial gap G andof an angle of the median line M of the insert with respect to the axisZ.

As shown in FIG. 16 the inserts 7 and 7' can be so arranged in the toolthat each median line M, M' is slightly slanted in different sensesforming angle ω to with the axis Z. Preferably, the angle ω does notexceed 8°. By virtue of the specific orientation of the componentcutting edges 50a and 50b described above, and by an appropriate choiceof the parameters characterizing this orientation, a proper compensationof radial components of the cutting force acting upon the insert may beachieved and, therefore, the drill stability during drilling is improvedproviding, at the same time, an effective chip splitting. As thecomponent cutting edges of each cutting edge slope in different senses,radial components FRa, FRa' of the cutting forces Fa, Fa' acting uponthe component cutting edges 50a and 50a' and radial components FRb, FRb'of the cutting forces Fb, Fb' acting upon the component cutting edges50b and 50b' of the inserts are seen to have different values anddirections with respect to the axis of rotation Z. Therefore, by virtueof optimization of shapes and positioning geometry of the inserts, animproved force equilibrium state in the radial direction may be achievedin the drill with two or more indexable inserts ensuring an improvedstability, particularly when entering or exiting from the workpiece, andreducing vibration during operation.

Though in the preferred embodiment of the present invention the drill isequipped with two inserts, it should be understood that the use of morethan two inserts or of only one insert is also possible. Thus, the useof only one insert in operation for enlarging a diameter D1 of theworkpiece W is shown in FIG. 17. The two component cutting edges 50a and50b slope in different directions so that the radial component FRa ofthe cutting force Fa acting upon the component cutting edge 50a isdirected towards the axis of rotation Z and the radial component FRb ofthe cutting force Fb acting upon the component cutting edge 50b isdirected away from the axis of rotation Z. Therefore, by virtue of theproper choice of mutual orientation of the component cutting edges aswell as of the mutual disposition of the cutting edges of the insertswith respect to the axis Z, the drill may be provided with an optimizedoverall balance of cutting forces, in particular of their radialcomponents.

With a drilling tool according to the present invention, drilling ofholes with diameters of 22-34 mm performed in low carbon steel at theappropriate feed of 0.15 mm/rev provided a surface finish of 1 μm Raafter the initial drilling operation. By virtue of wipers provided atthe main cutting corners, it was possible to combine drilling withinternal turning and to achieve a hole accuracy of IT5 and surfacefinish of 0.5 μM Ra. In order to achieve similar results by machiningwith conventional drills an additional tool would be required.

It should be noted that different alternative embodiments of the insertsemployed in the drill are possible. The inserts may differ in size orshape, depending upon the drill design and diameter. Each insert may beof square, rhomboid, parallelogram or octagonal rather than hexagonalconfiguration. The cutting edges of the insert max, have an asymmetricarrangement of the component cutting edges (FIGS. 6b). it should beunderstood that alternatively the component cutting edges 50a and 50bmay be oriented in the same direction as shown for example in FIG. 6c,or only one of the component cutting edges max, be slanted with respectto the median line while the other may be normal to this line (FIG. 6a).

If a cutting edge of the drill is long enough e.g. as shown in FIG. 18,it max, consist of two or more sets 200, 200' 200" of component cuttingedges, each set representing the cutting edge as shown in FIG. 6, i.e.consisting of two pairs of component cutting edges 50a-50b and50a'-50b', the component cutting edges 50a and 50a' of both pairs ofeach set merging in a common point 201 and the component cutting edges50a and 50b having extremities with respect to the point 201. Theextremity of the component cutting edge 50a and an innermost extremityof the component cutting edge 50b merge via a bridging section S whichis inclined with respect to a central line M' parallel to a median lineM of the insert so that the outermost extremity of the component cuttingedge 50b is closer to the line M' than the extremity of the componentcutting edge 50a ensuring that during the drilling action a lineparallel to a longitudinal axis of the drill and passing through theextremity of the component cutting edge 50a intersects the componentcutting edge 50b and the extremities overlap.

I claim:
 1. A metal drilling tool having an elongated body with alongitudinal axis and provided with at least one pocket at its workingend, at least one indexable cutting insert mounted in the or each pocketfor performing drilling action said insert comprising: a rake face,relief flank faces, a base, a plurality of cutting edges, each definedbetween the rake face and adjacent relief flank face and includedbetween two adjacent cutting corners, each of said cutting edgescomprising at least one set of component cutting edges, each setcomprising two pairs of component cutting edges, each pair comprising afirst and a second component cutting edge, the first component cuttingedges of both pairs merging in a common point, said component cuttingedges being oriented so that an extremity of the second componentcutting edge innermost with respect to said common point is closer to acenter of the insert than an extremity of the first component cuttingedge which is outermost with respect to said common point, saidextremities merging via a bridging section which is directed from saidoutermost extremity of the first component cutting edge towardsa centralline constituting or parallel to a median line of the insert passingthrough said common point so that said innermost extremity of saidsecond component cutting edge is closer to said central line than saidoutermost extremity of said first component cutting edge ensuring thatduring the drilling action a line parallel to said longitudinal axis andpassing through said outermost extremity of the first component cuttingedge intersects said second component cutting edge and said extremitiesoverlap.
 2. A drilling tool according to claim 1, wherein at least saidfirst component cutting edge is oblique with respect to said centralline of the insert.
 3. A drilling tool according to claim 2, whereinsaid first and second component cutting edges are oblique with respectto said central line of the insert.
 4. A drilling tool according toclaim 3, wherein said first and second component cutting edges areslanted with respect to said central line of the insert at acute angles.5. A drilling tool according to claim 2, wherein said first componentcutting edge is slanted with respect to said central line at an acuteangle and said second component cutting edge is slanted with respect tosaid central line at an angle which is not less than 90°.
 6. A drillingtool according to claim 1, wherein said pairs of component cutting edgesare arranged symmetrically with respect to said central line of theinsert.
 7. A drilling tool according to claim 1, wherein said pairs ofcomponent cutting edges are arranged asymmetrically with respect to saidcentral line of the insert.
 8. A drilling tool according to claim 1,wherein said relief flank face included between two adjacent cuttingcorners comprises a clearance component and a support component, bothsloping to the base at different angles.
 9. A drilling tool according toclaim 1, wherein the rake face of the insert is provided with a chipforming means in the form of a groove of variable dimensions.
 10. Adrilling tool according to claim 9, wherein the depth and the width ofthe groove formed along a component cutting edge which is radiallyoutermost with respect to said longitudinal axis of the drill are largerthan the depth and the width of the groove formed along the remainder ofthe cutting edge.
 11. A drilling tool according to claim 10, wherein aportion of said groove formed substantially along the first componentcutting edges and a second component cutting edge which is radiallyinnermost with respect to said longitudinal axis comprises two grooves.12. A drilling tool according to claim 1, wherein each cutting corner isprovided at both sides with a chamfer, said second component cuttingedge merging with said chamfer.
 13. A drilling tool according to claim1, wherein said at least one cutting edge is provided with anintermediate cutting corner located between said adjacent cuttingcorners of said insert and protruding beyond said adjacent cuttingcorners, as seen in an insert plan view, the first component cuttingedges of the both pairs of component cutting edges forming sides of saidintermediate corner.
 14. A drilling tool according to claim 1, whereinthe body of the drilling idol is provided with at least one additionalpocket at its working end for mounting thereto at least one additionalcutting insert for performing said drilling action.
 15. A drilling toolaccording to claim 14, wherein said cutting insert and said at least oneadditional cutting insert are of substantially the same design.
 16. Adrilling tool according to claim 14 or 15, wherein at least one of saidinserts is mounted in said pocket in such a manner that the median linethereof is inclined with respect to said longitudinal axis.
 17. Anindexable cutting insert for use in a drilling tool, comprising: a rakeface, relief flank faces, a base, a plurality of cutting edges, eachdefined between the rake face and adjacent relief flank face andincluded between two adjacent cutting corners, each of said cuttingedges comprising at least one set of component cutting edges, each setcomprising two pairs of component cutting edges, each pair comprising afirst and a second component cutting edge, the first component cuttingedges of both pairs merging in a common point, said component cuttingedges being oriented so that an extremity of the second componentcutting edge innermost with respect to said common point is closer to acenter of the insert than an extremity of the first component cuttingedge outermost with respect to said common point, said extremitiesmerging via a bridging section which is directed from said outermostextremity of the first component cutting edge towardsa central lineconstituting or parallel to a median line of the insert and passingthrough said common point so that said innermost extremity of saidsecond component cutting edge is closer to said central line than saidoutermost extremity of said first component cutting edge ensuringoverlapping of said extremities during a drilling action.
 18. An insertaccording to claim 17, wherein at least said first component cuttingedge is oblique with respect to said central line of the insert.
 19. Aninsert according to claim 18, wherein both of said first and secondcomponent cutting edges are oblique with respect to said central line ofthe insert.
 20. An insert according to claim 19, wherein said first andsecond component cutting edges are slanted with respect to said centralline at acute angles.
 21. An insert according to claim 19, wherein saidfirst component cutting edge is slanted with respect to said centralline at an acute angle and said second component cutting edge is slantedwith respect to said central line at an angle which is not less than90°.
 22. An insert according to claim 17, wherein said pairs ofcomponent cutting edges are arranged symmetrically with respect to saidcentral line of the insert.
 23. An insert according to claim 17, whereinsaid pairs of component cutting edges are arranged asymmetrically withrespect to said central line of the insert.
 24. An insert according toclaim 17, wherein said relief flank face included between two adjacentcutting corners comprises an upper clearance component and a lowersupport component.
 25. An insert according to claim 24, wherein saidsupport component is slanted with respect to said base at an angledifferent from a sloping angle of said clearance component.
 26. Aninsert according to claim 17, wherein the rake face of the insert isprovided with a chip forming means in the form of a groove extendingalong the cutting edge and having variable dimensions.
 27. An insertaccording to claim 26, wherein the depth and the width of the grooveformed along a component cutting edge which is radially outermost withrespect to a longitudinal axis of the drilling tool when the insert ismounted therein, are larger than the depth and the width of the grooveformed along the remainder of the cutting edge.
 28. An insert accordingto claim 27, wherein a portion of said groove formed substantially alongthe first component cutting edges and a second component cutting edgewhich is radially innermost with respect to said longitudinal axis,comprises two grooves.
 29. An insert according to claim 17, wherein eachcutting corner is provided at both sides with a chamfer, said secondcomponent cutting edge merging with said chamfer.
 30. An insertaccording to claim 17, wherein said insert is provided with anintermediate cutting corner located between said adjacent cuttingcorners and protruding beyond said adjacent cutting corners, as seen inan insert plan view, the first component cutting edges of both pairs ofcomponent cutting edges forming sides of said intermediate cuttingcorner.
 31. An insert according to claim 17, wherein said at least onecutting edge is formed with one set of pairs of component cutting edgesand wherein said central line is constituted by said median line.
 32. Aninsert according to claim 17, wherein said at least one cutting edge isformed with more than one set of pairs of component cutting edges andwherein said central line is constituted by a line parallel to saidmedian line and passing through said common point.
 33. An insertaccording to claim 17, wherein the extent of overlapping of saidextremities of the component cutting edges in each pair is such that,when the insert is mounted in the tool, a line parallel to alongitudinal axis of the tool and passing through said outermostextremity of the first component cutting edge intersects said secondcomponent cutting edge.
 34. A cutting insert for use in a drilling toolcomprising: a rake face, relief flank faces, a base, a plurality ofcutting edges, each defined between the rake face and adjacent reliefflank face and included between two adjacent cutting corners;at leastone of said cutting edges comprising at least one set of componentcutting edges, each set comprising two pairs of component cutting edgeswhich pairs are arranged at two sides of a central line passing througha common point of merger of the two pairs and constituting or parallelto a median line of the insert; each pair comprising a first and asecond component cutting edge merging through a bridging sectiondirected substantially towards said central line of the insert, saidcomponent cutting edges being oriented with respect to said central lineso that a radially innermost extremity of the second component cuttingedge is closer to the center of the insert and to said central line thana radially outermost extremity of said first component cutting edge. 35.An insert according to claim 34, wherein said first component cuttingedges form therebetween an intermediate cutting corner having an apex atsaid common point and protruding beyond adjacent cutting corners, asseen in an insert plan view.
 36. An insert according to claim 34,wherein in each pair of the component cutting edges, said firstcomponent cutting edge is slanted with respect to said central line atan acute angle.
 37. An insert according to claim 36, wherein both saidfirst and second component cutting edges are slanted with respect tosaid central line at acute angles.
 38. An insert according to claim 36,wherein said second component cutting edge is slanted with respect tosaid central line at an angle which is not less than 90°.
 39. An insertaccording to claim 34, wherein said pairs of component cutting edges arearranged symmetrically with respect to said central line of the insert.40. An insert according to claim 34, wherein said pairs of componentcutting edges are arranged asymmetrically with respect to said centralline of the insert.
 41. An insert according to claim 36, wherein therake face of the insert is provided with a chip forming means in theform of a groove extending along the cutting edge and having variabledimensions.
 42. An insert according to claim 41, wherein the depth andthe width of the groove formed substantially along a component cuttingedge which is radially outermost with respect to a longitudinal axis ofthe drilling tool, when the insert is mounted therein, are larger thanthe depth and the width of the groove formed along the remainder of thecutting edge.
 43. An insert according to claim 42, wherein a portion ofsaid groove formed along the first component cutting edges and a secondcomponent cutting edge which is radially innermost with respect to saidlongitudinal axis, comprises two grooves.
 44. An insert according toclaim 34, wherein both sides of each cutting corner is provided with achamfer, said second component cutting edge merging with said chamfer.45. An insert according to claim 34, wherein the extent of overlappingof said extremities of the component edges in each pair thereof is suchthat, when the insert is mounted in the tool, a line parallel to alongitudinal axis of the tool and passing through said outermostextremity of the first component cutting edge intersects said secondcomponent cutting edge.